The present invention relates to a lens structure which is incorporated in a camera having two or three focuses to realize a small-sized high-performance camera.
There are various kinds of technology for switching optical systems from one to another. A general kind of technology is to employ an additional optical element and use this element to change the focal distance of an optical system. Systems using such an additional optical element may be classified as a front type and a rear type. In the front type system a lens power-varying element is arranged in front of the lens system fixed to an optical apparatus, while in the rear type system it is arranged behind the lens system.
The front type system may be a combination of a standard lens (STAND) and a desired power-varying lens unit. An example of the power-varying lens unit is a telephoto lens (TELE) or a wide-angle lens (WIDE), and this power-varying lens is arranged in accordance with the need. The front type system may also be a combination of a wide-angle lens (WIDE) and a telephoto conversion lens (this combination provides a telephoto function). The front type system has to be inserted or removed each time the focal points are switched from one to another, which is very troublesome. In order to solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 63-43114 discloses a lens body structure capable of varying the focal distance thereof. The lens body structure disclosed in this reference comprises a fixed lens system, and a single front conversion lens group (a power magnifying lens group) rotatably arranged in front of the fixed lens system. With the front conversion lens group being inverted in direction by means of a power-varying lever, the three distances corresponding to telegraphic, standard and wide-angle modes can be easily attained.
A two-focus camera (generally referred to as a TELE/WIDE camera) may be an example of the rear type system. The two-focus camera comprises a standard lens (STAND) and a desired power-varying lens unit. The standard lens is fixed, and the desired power-varying lens unit is inserted in accordance with the need.
In recent years, a zoom lens is in wide use. The zoom lens comprises a number of lenses, and the focal distance of the zoom lens is changed in a stepless manner by changing the distances between the lenses.
A structure wherein a fixed lens optical system is combined with a power-varying lens system is disadvantageous in that the total number of lenses is inevitably large. In addition, the rear type system described above is disadvantageous in that an image observed through it is dark. Even in a wide-angle mode, the structure includes a group of lenses that are not used to attain the function. It should be also noted that a mechanism for inserting or retreating a conversion lens group is required not only for the lenses which are used to attain the function but also for the lenses which are not used then. This being so, it is hard to design the structure to be small in size.
The lens body structure disclosed in KOKAI Publication No. 63-43114 may be useful because it enables easy switching of a focal distance. However, this lens body structure employs a conversion lens group that is rotatable in front of the fixed lens optical system. It is therefore very hard to design the lens body structure to be small in size.
The zoom lens is inevitably large and heavy since its lens structure remains unchanged without reference to the mode of the zoom lens, i.e., even when it is in the mode corresponding to the standard focal distance. In order for a zoom lens to be designed to be small in size, hard-to-work lenses, such as an aspheric lens, have to be employed. This results in a high manufacturing cost.
In an ordinary type of photographing lens for a camera, a diaphragm is fixedly arranged at a single position inside the optical system of the lens. Since the position at which the diaphragm is arranged is an important factor for determining the optical characteristic of the lens, it is preferable that the position be determined with a high degree of freedom. In other words, the fixed position of the diaphragm makes it hard to design the photographing lens. In order to realize a high-performance photographing lens which employs a single diaphragm and which has characteristics similar to those of a single-focus lens, it is necessary to employ an increased number of lens elements, as a result of which the resultant photographing lens is heavy and large.
Accordingly, an object of the present invention is to provide a camera having an optical system which is light in weight and small in size and which has performance as high as that of a single-focus lens. Another object of the present invention is to provide a lens barrel for use in the camera.